1. Field of the Invention
The present invention relates to a method for fabricating a nonvolatile memory element and a nonvolatile memory element.
2. Description of the Related Art
The memory concepts (floating gate memories such as flash and DRAM) used to date are based on the storage of charges in inorganic, silicon-based materials. The technology for the storage of charges will encounter scaling limits in the foreseeable future. Therefore, alternative methods for storing information are increasingly being sought. In this case, the principle of resistive switching between two stable resistance states in organic molecules has proved to be promising.
Conjugated oligophenyleneethynylenes are used in this case as molecular wires in components (with nonlinear 1(V) curves for dynamic random access memories (DRAM)). Such organic molecules, such as e.g. amino-4-ethynylphenylbenzenethiol, are grown in a self-assembled monolayer (SAM). The open end of the molecular wires is subsequently provided with a top contact.
One problem in the deposition of self-assembled molecular monolayers is the adhesion or chemical binding of the organic molecule on the substrate. The semiconductor material (M. C. Hersam, R. G. Reifenberger; MRS Bulletin Vol. 29, No. 6 (2004) p. 385). A further problem is the compatibility of the process for fabricating the organic molecular memory element with the existing CMOS technology. No solution has hitherto been found either for the technological transfer of the molecular dimensions of the individual organic memory cells to a contact-connectable array comprising memory cells with the utilization of standard technologies, the intention being to retain as far as possible the advantage of the small dimensioning of the organic memory cell.
It has not been possible hitherto to satisfactorily realize either the coupling of the organic memory molecule to the substrate or integration of the organic memory molecule into a configuration that is compatible in conventional silicon technology in the dimension range of a few nanometers.
FIG. 1 shows a conventional realization of a molecular monolayer grown on gold with a gold contact (C. Zhou, M. R. Deshpande, M. A. Reed, L. Jones II and J. M. Tour; Appl. Phys. Left. 71(5), p. 611). In a membrane 1 made of SiN, a pore is opened by means of a plasma etching method (see FIG. 1a). Gold 3 is vapor-deposited into the well-type opening with a diameter of approximately 30 nm, whereupon organic memory molecules 4 are applied in the form of a SAM. The layer 4 made of organic molecules grown on the gold contact 3 is contact-connected by a further contact 2 made of titanium and gold (see FIG. 1b, an enlarged view from FIG. 1a). The enormous difference in the dimensions is clearly visible here. Dense packing of resistive memory cells has not been possible hitherto using this technology.
FIG. 2 shows an enlarged view of the resistive memory element. Gold contacts 6 are applied on a substrate 5. The contact between the molecule and the gold layer 6 is effected via a sulfide bridge of a thiol group of the organic molecule. As in this example, the top contact 7 may also be effected via a palladium contact or a metal-coated nanotube.
It has hitherto been possible to resolve the binding of organic memory molecules, such as e.g. amino-4-ethynylphenylbenzenethiol, via the thiol group to the substrate, which must be coated with a gold layer (J. Chen, M. A. Reed, A. M. Rawlett, J. M. Tour; Science, Vol. 2286 (1999), p. 1550Q). Complicated process steps have been required in this case, such as e.g. etching a bowl-shaped depression into the wafer, etching pores into a silicon membrane, filling in the gold bottom contact, and also mechanical polishing of the gold contact, as illustrated in FIG. 1. The integration of gold layers and the chemical mechanical polishing of this material are not possible without problems in the existing CMOS technology. In addition to the higher costs that occur as a result, the advantage of the small molecular dimensions and the high packing density possible can only be utilized to an unsatisfactory extent in the memory cell configuration realized to date. The high outlay and the low reproducibility in the application of titanium- or palladium-coated carbon nanotubes as top contacts on the organic memory molecules also constitute a problem.